1. Field of the Invention
This invention relates to a silicon nitride circuit substrate, a process for manufacturing the same, and a semiconductor device using the silicon nitride circuit substrate, and, particularly to a high thermal-conductive silicon nitride circuit substrate which is improved in mechanical strength and heat cycle resistance and which has excellent heat radiation characteristics, a process for manufacturing the same, and a semiconductor device with the same.
2. Related Art
A circuit substrate is manufactured by joining a ceramic substrate with a metal circuit plate. A direct joining process in which a ceramic substrate is directly bound with a metal circuit plate without using a binding material such as a brazing material is known as the joining process. This process comprises a step of producing an eutectic compound either between components contained in the metal circuit plate or between components contained in the metal circuit plate and the ceramic substrate and a step of binding the metal circuit plate with the ceramic substrate using this eutectic compound as a joining material. This joining process is effective in the case of using an oxide ceramic substrate, e.g. alumina. However, in the case where ceramics of a nitride, particularly a silicon nitride ceramic substrate as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 6-135771 is bound with a metal circuit plate, the wettability of the resulting eutectic liquid phase is reduced, which results in reduction of joining strength. Accodingly, if the obtained semiconductor device is started to load a repeat heat cycle on the joint portion, a crack is produced in the ceramic substrate at the position near the joint portion, which causes the circuit substrate to possess a reduced heat cycle resistance. This process is therefore an ineffective means for a nitride ceramic substrate.
The silicon nitride ceramic substrate is, however, a material possessing high thermal conductivity as well as high mechanical strength and thermal resistance. As a method for improving the bond characteristics, therefore, the present inventors have proposed, in Japanese Patent Application Laid-Open (JP-A) No. 7-250318, a measures of utilizing an intermediate layer containing aluminum and oxygen when joining a high thermal-conductive silicon nitride ceramic substrate with a metal circuit plate.
The method proposed in the above Japanese Patent Application Laid-Open (JP-A) No. 7-250318 is excellent in the joining characteristics. There is also a demand for further improving the heat cycle resistance and reliability of the joint portion. The present invention has conducted to produce a more reliable circuit substrate.